Before downgrading. have you tried:
- deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).
- switching to the default theme to rule out any theme-specific problems?
- re-saving your permalink structure or setting it to the default structure at Settings >Permalinks in your admin panel. If WordPress cannot automatically edit the .htaccess file, it will provide manual instructions after saving.